Method and means for thermographic reproduction



y 1963 T. G. WARTMAN ETAL 3,089,952

METHOD AND MEANS FOR THERMOGRAPHIC REPRODUCTION Filed Jan. 21. 1960 copy-paper? fivrz-wraes Tao/ms- Ci. Mar/m few/4204 0 A4424 3,089,952 METHUD AND MEANS FOR TI-ERMOGRAPHIQJ REPRGDUCTIGN Thomas G. Wartman, Mendota Heights, and Edward A. GMara, Mapiewood, Minn, assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn, a corporation of Delaware Fiied .Ian. 21, 1960, Ser. No. 3,755 7 Claims. (Cl. 250-65) This invention relates to the thermographic reproduction on heat-sensitive copy-paper of differentially radiation-absorptive graphic originals.

Thermographic reproduction processes are known in which a differentially radiation-absorptive graphic original, such for example as a printed page, is briefly intensely irradiated While in heat-conductive pressure-contact with a heat-sensitive copy-sheet. The selective absorption of radiant energy at the inked image areas results in the establishing of a corresponding heat-pattern at the graphic surface which, transmitted to the heatsensitive layer, produces a visible change.

The required pressure-contact between printed original and heat-sensitive copy-sheet may be provided by pressing the two together, for example between a transparent panel and a resilient compression cover or between a transparent flexible carrier belt and a revolving smooth rigid roll, during irradiation. The radiation must then pass through the transparent panel or belt in order to impinge on the inked surface of the original. High unit pressure during irradiation is required in order to assure uniform transfer of heat from inked image areas to the heat-sensitive layer, particularly at fold or crease lines. The necessity for transparent panels or belts which adequately transmit the high intensity irradiation While effectively resisting the severe stresses occasioned by the high contact pressures has, prior to the present invention, severely restricted the designer of therrnographic copying apparatus.

The present invention avoids the just-described restrictions and provides improved methods for accomplishing effective thermographic reproduction of typewritten, printed and other graphic original with simplified apparatus and employing novel heat-sensitive copy-paper, all as hereinafter described and illustrated.

In the drawing,

FIGURE 1 is a schematic representation of one illustrative form of copying apparatus as applied to the thermographic reproduction of graphic originals, and

FIGURE 2 is a sectional representation of a typical sheet of heat-sensitive copy-paper as employed with the apparatus of FIGURE 1.

The apparatus of FIGURE 1 consists essentially of a roll It) and a heated pressure platen II, in combination with a lamp source 12 of intense radiant energy, and along with suitable frame, power supply, control, and other elements as required. An ordinary household mangle or ironing machine provides an effective combination of roll and platen.

In operation, a heat-sensitive copy-sheet 13 and a graphic original 14 are congruently assembled, for example on a work-table 15, and are then passed between the roll and the heated shoe 11 under pressure supplied by spring 16. Temperature and pressure are sufficient to cause close smooth adherent contact between Patented May 14, 1963 original and copy-paper but without making any visible change in the heat-sensitive layer. The composite is then allowed to pass before the lamp 12, here shown to comprise a coiled filament 17 located along the interior focal line of a reflector 18 having a truncated elliptical crosssectiou. A suitable lamp having a filament ten inches in length may draw 1350 watts at 280 volts and provide a color temperature of about 2800 K. The composite is so guided that the exterior focal line of the reflector falls at the printed surface of the original, which is therefore briefly intensely irradiated as the composite passes the lamp 12. The thus treated composite 19 is then directed by guide 20 to a receptacle 21. After cooling, the copy is separated from the original by hand stripping. The copy-sheet separates cleanly, leaving no observable deposit on the surface of the original. The copy is a direct positive reproduction of the graphic original.

The same effect is obtainable by first combining the original and copy-sheet, and then separately irradiating the composite. Typical procedures involve first pressing the two sheets together between heated metal plates, or between a heated smooth metal roll and a compressible roll pressed thereagainst, and thereafter exposing the adherently held composite to brief intense irradiation from a progressively advancing linear source or a wide-area flash source or otherwise. In all cases the composite of copy-sheet and original during irradiation is held firmly together, with the elements in heat-conductive contact, by internal adhesion rather than by externally applied pressure.

Clear distinct copies are obtained by using a light source of adequate intensity and applying the radiation for a sufficiently brief interval. The requirements vary with the weight of paper, the ambient temperature, and various other factors. In general, best results are attained in the copying of originals typewritten on bond paper by employing a high intensity gas flash lamp rather than a hot filament source as heerinbefore described. Such a lamp may consist of a glass tube about ten inches long filled with xenon or other inert gas under reduced pressure and having an electrode at each end connected across an energy storage bank providing a 500 wattsecond discharge at a potential of 2600 volts DC. The extremely brief high intensity flash obtained from such a lamp minimizes or eliminates the small amount of blurring obtained on copies of such originals prepared using a hot filament source of radiation, the latter being better adapted for copying from originals or on copysheets having a heavier weight base stock.

FIGURE 2 represents a type of heat-sensitive copysheet which has been found particularly effective in the procedures here described. It consists of a transparent carrier or backing web 22, a heat-sensitive layer 23 containing at least two visibly interreactive components 24 and 25 in uniformly dispersedstate, and a thin, opaque, normally tack-free, heat-activatable surface coating 26.

As a specific illustrative example, the copy-sheet of FIGURE 2 is prepared with map overlay tracing paper, having a basis Weight of 25 lbs. per ream, as the carrier web 22. The heat-sensitive layer contains ferric stearate particles and methyl gallate particles uniformly distributed in a minor proportion of polyvinyl bu-tyral resin as an inert film-forming binder. The combination forms a visibly heat-sensitive coating which undergoes a distinct and immediate change in color when momentarily heated to temperatures of about 200250 F., e.g. by pressing the sheet briefly against a heated metal test bar. The opaque surface coating contains titanium dioxide pigment in an ethyl cellulose binder. Structures as thus defined have previously been described; see Clark Patent No. 2,813,043. In the present structure, however, the ethyl cellulose binder of the opaque surface coating is significantly modified by the incorporation therein of just sufiicient plasticizer material, for example triphenyl phosphate, to render the coating moderately tacky at temperatures slightly below the temperature at which visible reaction occurs while still being completely non-tacky at normal room temperature. Thus, a specific coating contains 166 parts by weight of titanium dioxide, 100 parts of ethyl cellulose, and 134 parts of triphenyl phosphate.

The thin surface coating just described is adequately tacky at temperatures of the order of 150 F. to provide an effective adherent bond to paper surfaces pressed thereagainst; and this bond is maintained as the composite is cooled. Separation of the sheets of the composite at the higher temperature results in splitting of the tacky layer and leaves a residue on the surface of the original. However it is found that, on first cooling to normal room temperature, the material increases sufficiently in cohesive strength so that the original and copysheet may be stripped apart without any splitting or transfer of the resinous layer and without any degrad tion of the printed surface.

The incorporation of the heat-activatable layer as a component of the copy-sheet itself, as just described in connection with FZGURE 2, is preferred, particularly in connection with front-printing techniques employing radiation-transmissive heatensitive copy-paper having a visibly opaque protective surface coating. The surface coating itself serves as the heat-aetivatable layer. The coating may be held at minimum thickness. However it is possible to obtain effective copies by applying the thin heat-activatable coating to the graphic original and then temporarily adherently attaching thereto a heat-sensitive copy-sheet free of such coating. With the coating applied over the printed surface, and used with radiationtransmissive copy-sheets, the system is suitable for multiple copying by front-printing. The invention is also applicable to back-printing, in which the heat-sensitive copy-sheet is temporarily adhered to the rear surface of a thin printed original during brief intense irradiation of the printed front surface. Tissue paper or other thin web impregnated or coated on both surfaces with the heatactivatable material is useful in providing the required temporary adherence between copy-sheet and original in either the front-printing or the back-printing position; but the use of such tissues involves additional time and expense in preparing the composite for copying, and has a tendency to cause blurring of image areas in the copy, and accordingly is not preferred.

Partially or fully radiation-absorptive copy-sheets appropriately releasably adherently bonded to differentially radiation-transmissive or -refiective graphic originals likewise provide useful composites for copying by shadow or reflex techniques using gas flash radiation somewhat as described in Miller et al. US. 2,844,733 and in Kuhrmeyer et al. US. 2,919,349. However these techniques do not ordinarily have the same rigid requirements of close contact between copy-sheet and original as do the more conventional front-printing or back-printing processes.

The specific heat-activatable coating or layer hereinbefore described contains a large proportion of pigment to provide opacity and contrast, but this component may be omitted where desired, or may be replaced by other suitable pigments, fillers, or other additives. Likewise the specific binder and plasticizer components may be widely varied so long as the resultant blend meets the requirements herein set forth. Other cellulose ethers and esters, various vinyl polymers, and many natural gums or resins (3. are suitable. Dioctyl phthalate or other suitable plasticizers may replace the triphenyl phosphate. The weight ratios of film-forming binder, plasticizer, and additives may be varied widely, in accordance with known principles, in obtaining the desired properties. In all cases there is provided a thin, normally strongly coherent and tackfree, temporary bonding layer which, at an elevated temperature somewhat below the copying temperature of the heat-sensitive copy-sheet, is activated to a pressure-tacky condition in which it readily lightly adheres under moderate pressure to paper and analogous surfaces. With such component the temporary assembling of the copysheet and graphic original in close heat-conductive association in a self-bonded composite becomes possible; and thermographic reproduction of the original on the copysheet is then easily achieved in the absence of continuous mechanically induced pressure-contact.

What is claimed is as follows:

1. Method for the thermographic reproduction of a graphic original, differentially absorptive of radiant energy, on a heat-sensitive copy-sheet, comprising: releasably adherently bonding together said copy-sheet and said original as a multi-ply composite; irradiating said composite with said radiant energy for a time and at an intensity sufiicient to cause reproduction of said original on said copy-sheet; and stripping said copy-sheet from said original.

2. Method for the thermographic reproduction on a heat-sensitive copy-sheet of a graphic original having image and background areas differentially absorptive of radiant energy, comprising: releasably adherently bonding said copy-sheet and said original together in heatcondnctive contact with a heat-activatable thin bonding layer non-tacky at room temperature and with said image and background areas in position for irradiation; irradiating said areas with said radiant energy for a time and at an intensity sufiicient to cause reproduction of said original on said copysheet; and stripping said copysheet from said original.

3. Method for the therrnographic reproduction on a heat-sensitive copy-sheet of a graphic original, said copysheet including a visibly heat-sensitive layer and a heatactiva-table surface coating non-tacky at normal room temperature and said original having image and background areas differentially absorptive of radiant energy, said method comprising: placing said copy-sheet with its said coating in contact with said graphic original and briefly pressing the two into close heat-conductive contact at an elevated temperature suificient to activate said coating to a lightly tacky condition but insufficient to cause a visible change in said heat-sensitive layer; irradiating said original with said radiant energy for a time and at an intensity sufficient to cause reproduction of said original on said copy-sheet; and stripping said copy-sheet from said original.

4. The method of claim 3 in which the copy-sheet is releasably adherently bonded to the rear surface of a thin graphic original having said image and background areas on the front surface.

5. The method of claim 3 in which the copy-sheet is transmissive of said radiant energy and is releasably adherently bonded to the front surface of said graphic original.

6. A heat-sensitive copy-sheet useful in the thermographic reproduction of graphic originals by the method herein described and including a chemically reactive, visibly heat-sensitive layer, the chemical reaction occurring within said layer on heating said sheet being responsible for the visible change, and a heat-activatable surface coating non-tacky and highly internally cohesive at normal room temperature while being sufficiently tacky, at an elevated temperature insufficient to cause said visible change, to form a releasable adherent bond with a firm paper surface on brief pressure-contact therewith, and

said coating comprising an organic polymer and a plasticizer therefor.

7. A heat-sensitive copy-sheet including a thin flexible carrier web, a visibly heat-sensitive layer capable of undergoing a rapid permanent visible change on heating to a minimum conversion temperature, and a plasticized organic polymer surface coating non-tacky and highly internally cohesive at normal room temperature and heatactivatable at an elevated temperature less than said minimum conversion temperature to a degree of tackiness sufficient to form a releasable adherent bond with a firm paper surface on brief pressure-contact therewith.

References Cited in the file of this patent UNITED STATES PATENTS Murray Apr. 11, 1950 Groak Nov. 4, 1952 Miller et a1. Dec. 22, 1953 Clark Nov. 12, 1957 Kuhrmeyer et a1. June 16, 1959 OMara Mar. 1, 1960 

1. METHOD FOR THE THERMOGRAPHIC REPRODUCTION OF A GRAPHIC ORIGINAL, DIFFERENTIALLY ABSORPTIVE OF RADIANT ENERGY, ON A HEAT-SENSITIVE COPY-SHEET, COMPRISING: RELEASABLY ADHERENTLY BONDING TOGETHER SAID COPY-SHEET AND SAID ORIGINAL AS A MULTI-PLY COMPOSITE; IRRADIATING SAID COMPOSITE WITH SAID RADIANT ENERGY FOR A TIME AND AT AN INTENSITY SUFFICIENT TO CAUSE REPRODUCTION OF SAID ORIGINAL ON SAOD COPY-SHEET; AND STRIPPING SAID COPY-SHEET FROM SAID ORIGINAL. 